Currently, there are two major types of IGBT devices: “punch through” and “non-punch through” devices. The “punch through” IGBT is normally fabricated on an epitaxial wafer, and “non punch-through” IGBTs on float zone (FZ) wafers. Typical epitaxial wafers used for IGBT manufacturing consist of two epitaxially deposited layers on an underlying substrate, a lightly doped top layer on top of a higher concentration buffer layer, on a substrate of opposite dopant type.
The buffer layer in the punch-through IGBT plays an important role in the performance of the device. It acts as a depletion stop layer under reverse bias and controls the injection efficiency of the backside anode in the forward conduction mode. The thickness and concentration of the buffer layer will affect the breakdown, forward conduction and the switching characteristics of the device. In general, the punch-through IGBT will have a better conduction (VCEON) and switching tradeoff than a non punch-through IGBT device for a given technology or manufacturing process. However, epitaxial wafers are more expensive than Float Zone wafers. For higher voltage devices (>600V), it becomes difficult to control the epitaxial layer (epi) resistivity and thickness uniformity.
As described in U.S. Pat. No. 6,707,111, a punch-through IGBT can be formed in a thin FZ wafers with a proton implant to create an N+ buffer layer.
It is also well known that IGBTs with a trench topology can be formed as described in U.S. Pat. No. 6,683,331, and will provide a lower on-state loss compared to a planar cellular or stripe topology.
The well-known figure of merit (FOM) for IGBTs is the trade-off between the conduction losses (VCEON) and the turn-off energy due to minority carrier recombination (tail current). This tail current increases the turn-off energy (EOFF). Any attempt to reduce the tail current amplitude and duration increases the forward voltage drop (VCEON) of the IGBT. Thus, trade-offs adjust IGBTs for different application requirements. PT IGBTs are available with various trade-offs to suit a wide variety of applications (speeds). NPT IGBTs have a low EOFF compared to that of PT IGBTs but, suffer from high VCEON, making them suitable for applications with switching frequencies >10 kHz.
It would be desirable to have an IGBT which has both lower VCEON and EOFF compared to the PT and NPT IGBTs and lower losses across a wide switching frequency range (4 kHz to 30 kHz). It would also be desirable to have an IGBT which can provide improved efficiency in a wide variety of applications with hard switching methods (low VCEON and EOFF) and also with resonant switching methods (low VCEON).